1. Field of the Invention
The present invention relates to a semiconductor element comprising a first layer with ZnSe as a body crystal and a second layer with ZnTe as a body crystal. More particularly, the invention relates to semiconductor light-emitting and semiconductor photoreceptor devices consisting of a II-VI family compound semiconductor.
2. Description of the Related Art
Recently, there is a demand for developing a blue or green, light emittable semiconductor light-emitting element as a light source for a high-density optical disk device, a high-density magneto-photo disk device, a full-color display device, a photo-chemical reaction processor, or a medical apparatus, for example.
Preferably, a II-VI family compound semiconductor consisting of at least one kind of II family chemical elements including zinc (Zn), magnesium (Mg), beryllium(Be), cadmium (Cd), mercury (Hg) and manganese (Mn) and at least one kind of VI family chemical elements including oxygen (O), sulfur (S), selenium (Se) and tellurium (Te) is used as a material constructing a green or blue light-emittable semiconductor light-emitting element. Particularly, crystal growth of a ZnMgSSe mixed crystal, a quaternary mixed crystal, is possible on a substrate consisting of GaAs which has excellent crystallinity and can be obtained easily. Also, the ZnMgSSe mixed crystal is known as a material constructing a guide layer and a clad layer of a semiconductor light-emitting element (Electronics Letters 28 (1992, p.1798, for example.)
Conventionally, a semiconductor light-emitting element using this kind of a II-VI family compound semiconductor, particularly, a semiconductor light-emitting element using a ZnMgSSe mixed crystal in a clad layer is generally constructed by sequentially laminating a clad layer consisting of an n-type ZnMgSSe mixed crystal, an active layer consisting of a ZnCdSe mixed crystal, a clad layer consisting of a p-type ZnMgSSe mixed crystal and a cap layer consisting of a p-type ZnSe sandwiching a buffer layer on a substrate consisting of n-type GaAs, and then, connecting a p-side electrode to the cap layer and an n-side electrode to the substrate. However, it is difficult to increase carrier concentration of a p-type ZnSe constructing the cap layer with this semiconductor light-emitting element. Thus, it becomes difficult to obtain ohmic contact with the p-side electrode.
To resolve the problem, a technology has been developed which consists steps of forming a super-lattice layer in which a p-type ZnTe layer and a p-type ZnSe layer are laminated alternately on the cap layer, forming an about 50 nm thick contact layer consisting of p-type ZnTe that can obtain high carrier concentration thereon, and then connecting a p-side electrode which consists of a palladium(Pd) layer, a platinum(Pt) layer, and metallic (Au) layer being sequentially laminated to the contact layer. This contact construction improves an ohmic contact characteristic with a p-side electrode substantially. Successive oscillation at room temperature has been achieved in a semiconductor light-emitting element with a Separate Confinement Heterostructure, SCH, construction in which a ZnCdSe mixed crystal is used as an active layer, a ZnSSe mixed crystal as a guide layer, and a ZnMgSSe mixed crystal as a clad layer (Jpn. J. Appl. Phys. 33 (1994) p. L938, for example.)
However, a conventional semiconductor element using this kind of contact construction causes a rapid increase in an operational voltage of 7-8V immediately after energization. To clarify its cause, a sectional construction of a semiconductor light-emitting element was observed through a transmission electron microscope, TEM. As shown in FIG. 1, it has been understood that high-density unmatched dislocation 130 of 3.2.times.10.sup.14 /cm.sup.2 occured on an interface between a super-lattice layer 120 and a contact layer 121. Also, it has been understood that a laminated layer defect 131 and a through-dislocation 132 were formed densely in the contact layer 121. In other words, a dislocation core of the unmatched dislocation 130 works as a hole-trap immediately after energization and a depletion layer is caused on interface between the super-lattice layer 120 and the contact layer 121, which may leads the increase in the operational voltage. The conventional semiconductor light-emitting element can not operate with a low voltage. Therefore, its life can not be extended, and its reliability cannot be improved.
As in the above description of a semiconductor light-emitting device using a II-VI family compound semiconductor, if a blue or green light-emittable semiconductor light-emitting element becomes practical, sufficient characteristics cannot be obtained with a photo-diode using a conventional silicon (Si.) Therefore, development of photo-diode using a II-VI family compound semiconductor has been demanded, but the ploblem of a contact construction with a p-side electrode must be resolved, just as in the case with semiconductor light emitting device.